This invention relates to a binder composition for shell molds, and more particularly to the preparation of shell molds for the casting of metals.
Precision casting of metal and other types of material in the molten state is used in many industries and, generally, such castings are made in expendable molds. There are three general types of processes for making the expendable molds and these may be classified as the "lost wax," the single investment and the double investment processes. These all have one thing in common; they are one-use molds in which the mold is generally destroyed in removing the casting therefrom. To provide an economical process, a master mold or pattern is initially prepared from which refractory molds are made by one of the above processes.
In the manufacture of precision castings by investment shell casting techniques, disposable patterns are made from waxes, plastics, frozen mercury and other materials which readily may be removed from the mold. The investment cycle consists of making the pattern by injecting the pattern material into the die and gating the pattern to a central sprue to form a pattern cluster.
Generally, the pattern cluster is dipped into an agitated slurry of the coating composition, drained, stuccoed while still wet with particulate mold material and dried. The dipping, draining, stuccoing and drying sequence is repeated the desired number of times, depending on the thickness and strength of the shell mold desired.
Thereafter, the disposable pattern is removed by methods such as melting or solvent treatment and the mold cured by firing at a temperature sufficient to remove the volatiles and provide adequate bonding. The molds are then heated and filled with molten metal and after cooling, the castings are removed from the sprue and finished in the usual manner.
The preparation of shell molds has been described in U.S. Pat. No. 3,270,382 to Emblem et al in which two different coatings are applied alternately to an expendable pattern. One coating consists of a gel-forming acid hydrolyzed alkyl silicate solution and the other coating consists of a gel-forming basic silicate ester. The acid hydrolyzed solution and the basic silicate ester of the applied coatings each act upon the other so as to cause gelation of the other.
U.S. Pat. No. 3,079,656 to Emblem et al, describes the preparation of aminosilicates by an interchange reaction between an isopropyl silicate and a mixture of aminoalcohols, while removing the alcohol produced as the result of the interchange reaction. The aminosilicates may be combined with a refractory material and applied to an expendable pattern. The coating is gelled by the addition of water. Likewise, aminoalkyl silicates have been disclosed as binders in U.S. Pat. No. 3,112,538 to Emblem et al. The binder is combined with fine refractory powders to form a slurry which is applied to a wax pattern. Each coating is allowed to partially harden before the next is applied. The coated pattern is then hardened for 24 hours before the wax pattern is removed.
Binders containing a mixture of an alkyl silicate and an aminoalkyl silicate have been described in U.S. Pat. No. 3,329,520 to Emblem et al. These binders are combined with a refractory material and water and then cast into the desired shape.
Emblem et al also disclose in U.S. Pat. No. 3,314,806 a binder composition containing an isopropyl silicate, water and a base as an accelerator for the hydrolysis and gelation of the silicate.
A paint composition containing a mixture of organic silicates and an amine having a pKa value above 7.8 is described in Australian Pat. No. 163,467. However, when the composition contains an amine such as monoethanolamine, the mixture must age for at least 48 hours and up to fourteen days before it can be used as a protective coating on sand cores.
The process of this invention constitutes an improvement in investment casting over that disclosed in the art. For example, the binder composition of this invention does not require an aging period, i.e, the composition is ready to use immediately after mixing the ingredients. Moreover, the resulting shell is ready for use after air drying for from one to three hours. Also, the binder composition of this invention has a better shelf-life and longer pot-life than similar aminosilicate compositions. In addition, the binder composition of this invention gels in the presence of atmospheric moisture and does not require adding additional water to the binder composition. Furthermore, the process of this invention utilizes a single binder composition to prepare shell molds, whereas many of the shell molds prepared heretofore required at least two different types of binder compositions.
Therefore, it is an object of this invention to provide a process for preparing shell molds. Another object of this invention is to provide a process for preparing shell molds which utilizes a single binder composition. Another object of this invention is to provide a process for preparing shell molds which may be used after air drying for from 1 to 3 hours. A further object of this invention is to provide a binder composition which does not require the addition of water to cause gelation thereof. A still further object of this invention is to provide a binder composition which does not require removing the alcohol resulting from the esterification of an organosilicate and an aminoalcohol.